He provided a solid and serviceable artwork, but often it lacked the majesty, I guess, one would expect for Superman. To me it really depended on the inkers he got, and in the 70s unfortunately that usually meant Chiramonte or Colletta, neither of whom inked like they cared, which only underscores Swan's flaws and didn't enhance his strengths.
When, however, he had an inker that had a strong character of their own, his work became an excellent framework. The Moore story is a case in point: Perez and Schaffenberger. And then there was Murphy Anderson, usually considered his best inker. (though there's something about Anderson's way of rendering faces that kind of creeps me out) And then there was the truly great work Al Williamson did on top of his pencils. All of these strong cartoonists in their own right. (Anything Schaffenberger inked, visually became Schaffenberger's entirely) The inker of Swan really had to bring something of their own to the table. It'd have been interesting to see what Tom Palmer might have done.